The effect of partial replacement of ordinary Portland cement (OPC) with nano-clay (NC) on the performance of physico-chemical, mechanical, hydration characteristics and microstructure of composite cement pastes containing ordinary Portland cement (OPC) and ground blast-furnace slag (GBFS) was studied. Initial and final setting times are accelerated by the partial replacement of OPC with NC. The values of compressive strength, bulk density, chemically combined water content and gel/space ratio of composite cement pastes increase by using NC up to 6 mass%, then decrease by further replacement up to 8 mass%. The free lime contents of composite cement pastes increase from 1 day up to 3 days, then decrease from 7 days up to 90 days. NC activates the hydration reaction of OPC and GBFS. SEM micrograph of mix M4S40 (54% OPC + 6% NC + 40% GBFS + 1%SP) deposits the presence of micro- and nano-crystalline CSH gel with dense interlocking close linkage structure composed mainly of larger crystalline hydrates. It can be concluded that 6 mass% NC is the optimum replacement level.